During electronic device manufacturing, undesirable materials may build up on the edge of a substrate. The materials may include dielectrics, photoresist and metals used in IC manufacture. Therefore, it may be desirable to clean or polish the bevel and edge of the substrate to remove these materials. It may also be desirable to determine when a particular or desirable amount of polishing has occurred. Systems, methods and apparatus for measuring the film thickness on the edge of a substrate during polishing are needed.